A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, it is necessary to measure parameters of the patterned substrate, for example the overlay error between successive layers formed in or on it. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. One form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
Existing apparatus use a specific target to determine the overlay error or critical dimensions. The target usually comprises a grating and the target area is of a sufficient size to be larger than the radiation spot with which it is illuminated. Consequently, reconstruction of the grating structure is relatively simple because the target can be considered to be periodic and of infinite extent. As the target does not form part of circuit pattern it occupies an area of the substrate, which cannot then be used as part of the circuit pattern. However, space on the substrate is at a premium, and it is therefore desirable to reduce the area occupied by targets.
If the target area is reduced, then the edges of the target will fall within the edge of the radiation spot with which it is illuminated. Thus, the grating cannot be considered infinite and can be considered to be an infinite grating multiplied by a window function, with the edges of the target forming the window function.
If the grating is no longer infinite, then each diffraction order is spread, and is formed by the Fourier transform of the shape of the target as a whole. The diffraction pattern that results from the finite grating can be thought of as obtained from the diffraction pattern of the infinite grating that has been convoluted with the Fourier transform of the window function of the finite target. The smaller the size of the target, the larger the spread of each diffraction order. As a consequence of the spread of each diffraction order, the diffraction orders begin to overlap, which makes reconstructing the grating more difficult and may result in errors in the resulting calculations.
A conventional target comprising a grating is depicted in FIG. 5a of the accompanying Figures. FIG. 5b depicts the diffraction orders resulting from the grating. As can be seen in the figure, the spread (or streaking) of the diffraction orders is greatest in the x and y directions, and the different diffraction orders are also oriented along the x direction. Thus, there is some overlap between the different diffraction orders along the x (or horizontal) direction.